An ink-jet printer is a type of non-impact printer which forms characters and other images by controllably spraying drops of ink from a print head onto a recording media, such as paper. The print head ejects liquid ink through multiple nozzles in the form of drops which travel across a small air gap to land on the recording media. The drops are very small as ink-jet printers commonly print within a range of 180 to 600 dots per inch (dpi). The ink drops dry shortly thereafter and combine to form the printed image.
There are two major classes of ink-jet printers: drop-on-demand and continuous. Drop-on-demand ink-jet printers eject ink only when ink is required for printing, whereas continuous stream ink-jet printers propel ink in streams and deflect charged drops either toward or away from the media. This invention is particularly directed to drop-on-demand ink-jet printers.
A thermal ink-jet printer is a drop-on-demand printer which uses heat dissipation to form and eject ink drops. A thermal ink-jet print head has multiple drop generators which are used in parallel to increase printing throughput. Ideally, there is a drop generator for each nozzle. Each drop generator includes an ink chamber, a nozzle orifice, and a heating element disposed at the nozzle. Ink droplets are ejected from individual nozzles by localized heating of the associated heating element for the selected nozzles. An electrical current is passed through the associated heating element to heat it up. The current is typically supplied in individual energy pulses of sufficient duration and magnitude to cause deposition of an ink drop. The heat dissipated from the heating element during a "firing pulse" vaporizes a tiny volume of ink in the associated chamber causing the ink to volumetrically expand. This forces unevaporated ink through the respective nozzle toward the recording media. Contraction of the vapor bubble contributes to breakoff of the ejected ink to form a drop which continues its path to the media.
One problem associated with ink-jet printers concerns the amount of ink deposited from the print head during formation of each drop. The quantity of deposited ink, commonly referred to as "drop-volume", significantly contributes to print quality. Low drop-volume results in poor quality images which appear faint or washed out. Conversely, high drop-volume yields poor quality images which appear too dark or have poor resolution. High drop-volume also increases the amount of time necessary for the image to dry.
Drop-volume is dependent on the temperature of the print head which, in turn, is based upon the amount of energy applied during the firing pulses. The amount of energy applied to nozzle heating elements needs to be controlled to produce the desired drop-volume for ensuring high quality images. If too little energy is applied, the print head is cooler and deposits less ink, thereby resulting in a low drop-volume. If too much energy is applied, the print head becomes exceedingly warm and deposits too much ink, thereby resulting in a high drop-volume. Additionally, excessive high energy can damage the print head.
The deposition characteristics of an individual droplet ejected from a drop generator can be controlled by manipulating the amount of energy applied to the associated heating element during a firing pulse. There is a threshold point of energy above which a drop is ejected, and below which no drop is ejected. The threshold point can be used to derive an optimum operating range for drop generator. Ideally, the drop generator is operated above its threshold point at a point that deposits the desired drop-volume.
Designers have proposed various techniques to derive and control the appropriate amount of energy which causes ejection of an optimal droplet. U.S. Pat. No. 4,872,028 to Lloyd, and also assigned to Hewlett-Packard Company, describes a thermal ink-jet print system that has a piezo-electric drop detector which is used to help determine an optimal drive pulse. The drop detector is located within a maintenance station to the side of the media path.
During a startup routine, the print head is moved to the maintenance station to undergo an operating energy calibration test. A test generator supplies a series of energy pulses of fixed amplitude and successively increasing pulse widths to the print head. The test pattern contains both pulses having insufficient energy to produce a detectable drop (i.e., below the threshold point) as well as pulses having enough energy to cause deposition of a detectable drop. When ejected, the drops collide with the piezo-electric membrane of the drop detector causing generation of an electric signal indicating that a drop was ejected. The feed back from the piezo-electric drop detector is analyzed by a microprocessor in relation to the energy pulse test pattern to derive an optimum operating energy.
U.S. Pat. No. 4,509,057 to Sohl et al. describes a technique for automatically calibrating a drop-on-demand ink-jet print head using an optical drop detector. The optical drop detector includes a light emitter which directs a light beam toward a light detector. The light beam is positioned adjacent to the print head nozzle so that a properly ejected droplet passes substantially through the light beam, thereby interrupting it. The interruption to the light beam is sensed by the detector to monitor when an ink droplet is ejected. In addition to detecting when a print head is firing, the optical detector can help determine horizontal errors in drop position by relating the position of the nozzle to the amount of light blocked by the droplet. Additionally, the velocity of the droplet can be determined by measuring the amount of time elapsed between droplet ejection and droplet detection.
The piezo-electric and optical drop detectors of prior art solutions have a significant drawback in that they can become contaminated with ink. With respect to the piezo-electric drop detector, the membrane is directly coated with ink when the droplets impact the membrane during testing. For the optical drop detector, it is closely situated near the droplet path and can become covered with ink aerosol which consists of tiny droplets of ink that are discharged during ejection of an ink droplet. Over time, the ink aerosol accumulates and begins to cover the optical detector or emitter, impairing or blocking the light beam. As a result of this ink contamination, there is a possibility that the performance of a piezo-electric detector or an optical detector will degrade over time.
It is therefore an object of this invention to provide a drop-on-demand ink-jet printer having a drop detection system for determining an optimal operating energy that will not be degraded by ink contamination.